This invention relates generally to direct conversion receivers and more particularly to filters used in such receivers to remove unwanted DC signals produced by the conversion process or generated by elemental circuit variations.
As is known in the art, a direct conversion receiver mixes a received radio frequency signal directly with the carrier frequency of such signal to thereby extract information in the received signal modulating the carrier frequency. One method used to remove unwanted DC components produced by the direct conversion mixing process is to pass the output of the mixer through a DC blocking capacitor, i.e., a high pass filter, prior to subsequent amplification. In many applications it is desired to fabricate the receiver as an integrated circuit; however, the size of the capacitor, in many applications, would be so large that its fabrication in the integrated circuit would be impractical.
As is also known in the art, one technique suggested to provide this DC blockage is to use a filter having a high gain amplifier with a low pass filter in negative feedback therewith to produce a high pass filter.
In accordance with one feature of the invention, a direct conversion receiver is provided having a homodyning section fed by: a received radio frequency signal having a carrier frequency; and, a reference signal having the carrier frequency. A high pass filter is coupled to an output of the homodyning section. The high pass filter includes an amplifier and a low pass filter coupled in a negative feedback arrangement with the amplifier. The low pass filter is adapted to have the cutoff frequency thereof switch from a high cutoff frequency, during an initial phase of operation, to a subsequent lower cutoff frequency during a normal operating phase.
The high pass filter acts as a dc offset correction loop which eliminates the serial effect of many amplifier sections on dc offsets arising within components while maintaining a sufficiently low cutoff frequency to avoid adversely impacting information integrity at higher frequencies. The plurality of high pass filter sections also enables the integration of the needed capacitors thus minimizing external components and connections.
In accordance with another feature of the invention, a high pass filter of a direct conversion receiver is provided which includes, in a low pass filter feedback section thereof: a capacitor; and, a resistor section coupled to the capacitor. The resistor section includes a switch for changing the resistance of the resistance section from a first, relatively low value, during the initial phase, to a second higher resistance during the subsequent, normal operating phase.
In accordance with another feature of the invention, a direct conversion receiver is provided having a homodyning section fed by: a received radio frequency signal having a carrier frequency; and, a reference signal having the carrier frequency. A high pass filter is coupled to an output of the homodyning section. The high pass filter includes an amplifier and a low pass filter coupled in a negative feedback arrangement with the amplifier. The low pass filter has a capacitor and a switching section for rapidly charging the capacitor during an initial, pre-charge phase.
In accordance with still another feature of the invention, a low pass filter is provided. The low pass filter includes a differential amplifier having a first pair of transistors and a second pair of transistors. Each one of the transistors has a first electrode, a second electrode and a control electrode. The control electrode is adapted to control a flow of carriers (e.g., current) between the first and second electrodes. The control electrodes of the first pair of transistors provides a first input for the differential amplifier and the control electrodes of the second pair of transistors provide a second input for the differential amplifier. A current source is coupled to the first electrodes of the first and second pair of transistors. A capacitor has a first electrode coupled to the second electrode of a first one of the transistors in the first pair of transistors and a second electrode coupled to the second electrode of a first one of the transistors in the second pair of transistors. A pair of switches is included. A first one of the switches is adapted to couple the second electrode of the second transistor in the first pair thereof to either the first electrode of the capacitor, during an initial pre-charging phase; or, to the second electrode of the capacitor during a subsequent, normal operating phase. The second one of the pair of switches is adapted to couple the second electrode of the second transistor in the second pair thereof to either the second electrode of the capacitor, during the initial pre-charging phase; or, to the first electrode of the capacitor during the subsequent normal operating phase.
In accordance with one embodiment of the invention, the first and second transistors in each of the pair thereof is adapted to pass current from the current source through the first and second electrodes thereof with different current levels.
In accordance with one embodiment of the invention, a first resistance is provided between the current source and the first electrode of the first transistor in the first pair of transistors and a second resistance is provided between the current source and the first electrode of the first transistor in the second pair of transistors.
In accordance with one embodiment of the invention, an additional switch for changing the resistance of the first and second resistances from a lower value to a higher value when the filter changes from the initial pre-charging phase to the subsequent normal operating phase.